Turbine generators used by the electric utility industry generate high temperatures in various generator components. Of particular interest to the present invention, undesirable heat is generated in the stator windings of such generators and must be removed to keep the stator windings from overheating. To provide the necessary cooling to the stators windings in these turbine generators, a variety of techniques have been employed. One technique incorporates hollow individual conductor strands in the stator coil through which a liquid coolant circulates (see e.g., Collins, U.S. Pat. No. 3,614,493, which is hereby incorporated by reference). The coolant collects heat from the conductor strands, carries it away from the stator coil, and releases it.
A stator coil used in such generators is generally comprised of a bundle of individual conductor strands, some of which may have hollow cores, that are collected together to form a single stator coil. At the lead end of the stator coil, where the conductor strands terminate, it is necessary to complete the electrical and coolant flow circuits. For these circuit completion purposes, a conductive header is used that makes the electrical connection while providing a conduit for the coolant to flow through the stator coil. Thus, coolant flows through the header and into the hollow conductor strands of the stator coil where heat is collected. After collecting the heat, the coolant flows out through a header at the other end of the stator coil.
Because the stator coil and the header are separate components, they must be joined together at their interface. Typically, the stator coil is joined to the header with a conductive braze that provides electrical conductivity between the header and stator while providing a fluid-tight seal. In recent years, the electric utility industry has experienced problems with water leakage in turbine generators that use water-cooled stators. Specifically, water leakage around the area of the stator coil to header braze joints has caused generators to fail.
Some water cooled stator coils are susceptible to leaks due to a corrosion process through the consolidated braze. This leads to wetting of the internal coil, subsequent degradation of the groundwall insulation and electrical failure of the coil through the degraded groundwall insulation.
Leaking water can damage insulation and over time cause an electrical failure which can be costly to repair. Leaks have been attributed to poor initial brazes, porosity in cast components, water chemistry issues and crevice corrosion. Crevice corrosion refers to a mechanism which results in corrosion tunneling through the clip-to-strand, or consolidation braze.
Thus, there is a need for a method of repairing the stator coil and header interface to stop the leakage and prevent the likelihood that the leak will recur.